Mu is a bacterial virus that uses genetic transposition to integrate and replicate its DNA. Mu is the most active transposable element known and during the lytic cycle Mu uses the E. coli replication machinery to transpose to 100 different sites in the bacterial chromosome within an hour. The viral elements necessary for transposition include DNA sequences at both ends of the virus and the products of Mu genes A and B. Our long range objective is to dissect the biochemical mechanism of Mu transposition. We recently developed an in vitro system for carrying out efficient Mu transposition-replication. We plan to use this in vitro system to characterize transposition intermediates and to purify Mu transposition proteins. In the course of these studies we will identify the origin(s) of Mu replication, define the mechanism of initiation of DNA synthesis at the origin, and determine the biochemical properties of the proteins that carry out transposition. The process of transposition is now recognized in the dissemination of drug resistance genes and plasmids in bacteria, in DNA rearrangements of prokaryotic and eukaryotic chromosomes, and in the developmental cycle of oncogenic viruses. However, the biological impact of genetic transposition is only beginning to be appreciated. These studies will significantly advance our knowledge of the functions of the proteins and genetic sequences involved in this phenomenon.